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gabriel
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« on: September 07, 2017, 09:07:57 21:07 »

Hello:
I need to measure moderate temperatures between 5C and 50C and do some commands with a microcontroller. I founded that LM35 is the most appropriate, (can be directly connected to the micro without additional components, linear, etc...), some colleagues advised me to use NTC thermistors, and said that it's most widely used. I think that thermistors should be mounted in a voltage divider and then do some corrections in program, so not better than LM35. For this range of temperature, is there any advantages for using thermistors instead of LM35?
other question: some sensors should be remotely mounted far from the board (about 30 meters), is there any precaution for using LM35 at that distance?

Any help is appreciated.

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Wizpic
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« Reply #1 on: September 07, 2017, 09:36:03 21:36 »

30 Meters I would say yes, major noise issue.
I don't like LM35.  may be over kill but I would use a DS18B20 or DHT22 with an NRF24L01 using Arduino and transmit the data rather than using 30 meters of cable, Or even use an LM35 with the same set up if you wanted to.
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rtm
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« Reply #2 on: September 07, 2017, 10:04:07 22:04 »

I've used both LM35 and DS18B20. I can say that DS18B20 is better choice for me. I had a lot of problems with noises for LM35, so it took additional efforts to solve it in program. Using of DS18B20 is much simpler, and program for its digital 1-wire port is not so complicated at all.
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PICker
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« Reply #3 on: September 07, 2017, 10:23:29 22:23 »

Please, read this appnote from Microchip, you can order free samples...
http://ww1.microchip.com/downloads/en/DeviceDoc/21895a.pdf
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gabriel
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« Reply #4 on: September 08, 2017, 12:08:46 12:08 »

thanks for reply.
Do you think that DS18B20 can be connected 30 meters away?
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Wizpic
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« Reply #5 on: September 08, 2017, 01:34:10 13:34 »


You really should not exceed about 1.5 meters tops. This is similar to the i2c bus length; if you exceed about one meter then you will have intermittent problems due to capacitance issues and|or noise. words to the wise
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Signal
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« Reply #6 on: September 08, 2017, 03:37:51 15:37 »

Do you think that DS18B20 can be connected 30 meters away?
According to https://www.maximintegrated.com/en/app-notes/index.mvp/id/148 30 meters should not be a problem.
Some discussions from other forums reported a fully successful 30 m connections with DS18B20 without special master drivers.
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gabriel
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« Reply #7 on: September 08, 2017, 09:43:27 21:43 »

searching wikipedia for "one wire", I found:
Quote
Up to 300 meter long buses consisting of simple twisted pair telephone cable have been tested by the manufacturer. It will however require adjustment of pull-up resistances from 5 to 1 kΩ.
interesting
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« Reply #8 on: September 09, 2017, 03:33:15 15:33 »

Although both I2C and Dallas single wire are both open collector so probably less sensitive to noise than say SPI, I suspect that the inherent simplicity of Dallas single wire is even less sensitive than I2C.

If you have an open collector GPIO on your ucontroller,  the 18b20 will work nicely on it.
« Last Edit: September 09, 2017, 03:38:17 15:38 by towlerg » Logged

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